Concentrations of dissolved hydrogen in steel, as low as 2 ppm, are known to be detrimental in many critical applications. Typical concentrations of hydrogen in molten steel are on the order of 3-17 ppm. Large forgings and thick rolled plate products are particularly susceptible to hydrogen embrittlement which may lead to flaking and cracking. Several industry and government material specifications require median dissolved hydrogen contents to be below 2 PPM for certain applications and grades of steel.
There are several processes known for the removal of hydrogen in the art; among these are the following:
Vacuum treating of steel is the only commercially acceptable method of reducing dissolved hydrogen (H) to very low levels in the molten state. This method exposes the molten metal to a vacuum having a pressure on the order of 0.5 torr. This is a mechanical means of inducing H.sub.2 out-gassing. Hydrogen levels below 2 PPM can be obtained if the process is performed correctly. This type of hydrogen removal is taught in U.S. Pat. No. 3,060,015.
Another method of degassification is the slow cooling of steel products, such as blooms and slabs, to allow the hydrogen to diffuse out of the steel naturally. Such slow cooling, however, extends processing time, ties up inventory and require extreme care to ensure reliable results.
There are also processes using large amounts of gas sparging to help reduce the dissolved hydrogen content. Generally, there are referred to as subsurface pneumatic refining processes, e.g., argon/oxygen decarburization (AOD). Unfortunately, these processes are not a reliable or effective method of producing low H metal. Disclosure of these subsurface pneumatic refining processes and their modification to produce a low H metal is provided in U.S. Pat. No. 4,451,288.
Reaction of halogen-containing compounds with molten steel in the "killed" state to chemically remove dissolved hydrogen has also been investigated. This type of chemical removal process is taught in U.S. Pat. No. 3,199,976 and Japanese Patent Application 1981-125,276. Unfortunately, the reaction rates and process efficiencies for this process are low which renders them commercially unacceptable.